The data collected by a satellite are only as good as the signal it sends back to Earth, and the signal it sends back is only as good as the antenna that sends it. Modern satellites come equipped with various sorts of antennas, all of which are designed to send and receive data by transmitting and interpreting pulses of electromagnetic radiation. Most satellites operate in a portion of the microwave spectrum known as the Kᵤ band, which spans wavelengths ranging from 1.67 to 2.5 centimeters and frequencies between 12 and 18 gigahertz.
In a new study, Turkmen and Secmen design, model, and fabricate a new type of omnidirectional and circularly polarized slotted antenna that improves on existing designs in a number of ways. The word “omnidirectional” is used to describe antennas that transmit their signal isotropically, meaning the pattern of radiation is the same no matter where the receiver is placed relative to the transmitter. Although perfectly isotropic transmission remains impossible, researchers can manipulate the signal in several ways to reduce its directionality. Omnidirectional antennas have several advantages, most notably in their ability to transmit around landforms such as mountains or, in the case of satellites, around the curvature of Earth, allowing researchers to maintain constant contact with the orbiter and detect any faults.
Similarly, circular polarization of the signal allows the satellite and the ground station to maintain communication even if the satellite rotates relative to the receiver or if disturbances in the atmosphere cause the electromagnetic signal to rotate as it travels to and from the ground.
Here the authors propose a new antenna designed to create the truest omnidirectional radiation pattern yet. It uses a special waveguide (a hollow structure that controls and aims the electromagnetic radiation) that transitions from a rectangular shape to a cylindrical one (see the image above). Like a sound wave traveling through an organ pipe, the satellite signal propagates through the wave guide, and the unique shape coaxes the signal into a pattern known as the TM01 mode, which also improves the omnidirectionality of the signal.
To improve the signal’s quality even further, the researchers placed nonidentical antennae array slots in a geometrically symmetric pattern along the waveguide (see the image above). This modification was done to decrease the gain variation in the signal in the azimuthal plane in a wider frequency bandwidth. Gain describes how much a signal is amplified, and low variations in gain are crucial for achieving an omnidirectional radiation pattern. The end result, the researchers say, doubles the bandwidth of the satellite at the 12-gigahertz frequency. (Radio Science, https://doi.org/10.1029/2018RS006635, 2018)
—David Shultz, Freelance Writer